[SOLVED] Positive to negative voltage converter

[infernal669]

Hello, I'm trying to build a power supply with +/-5v. I got an LM2575-5 and build the circuit from the datasheet.
I'm using a laptops power supply as input, its rated at 19V, 4.74A.
I have an LED connected on the output but my circuit doesn't seem to work.
I came across this circuit Dual 5V Power Supply Mini Board so i tried to use a bigger coil but all i got was some smoke coming out from the coil and the output voltage was like 1.23v.
Any tips plz?

 

[RetroTechie]

Welcome here, infernal669!

You're using LM2575, but following application note from LM2576 datasheet? LM2575 datasheet (downloaded from National Semiconductor) has a coil with bigger self-inductance there (100 uH). And another type diode, if you stay under 1A output it should work the same here I think.
What value (inductance) coil are you using, and what max. current is it rated for? And what type diode?
What value capacitors are you using?
Can you make a picture of how you wired things up?
Are you sure the diode and LM2575 are still okay after that smoking coil :shock: incident? Do you have replacements?
Have you checked (read: measured) that regulator gets the proper input voltage from laptop power supply?

 

[infernal669]

Actually i made a mistake on my attachement, the correct is here. https://obrazki.elektroda.pl/5_1324241614.jpg
The point is that i tried this circuit in the beggining with 9V 500mA input but it just didnt work,the output voltage was really low... Then i found out this Dual 5V Power Supply Mini Board so i tried to change the capacitor and the coil on the output. I used 1000μF capacitor and a bigger coil(i dont know the value ) didnt work so now i tried with more voltage on the input cauz i read somewhere that minimum input voltage for -5v output is 12v.
Also what confuses me is that the same materials for the +5v dc converter works pretty good. So why would a coil start burning? Too much current flowing through it? These guys that made this Mini Dual 5v PSU are using different capacitors and coils than the circuits in the datasheet. What coil value would you suggest me to use? Diodes i use are 1N5819 same as the datasheet circuits. I have the circuit on a breadboard but im 100% sure that i wire it correctly.
Thnx for your help.

 

[RetroTechie]

LM2575 datasheet says: "For an input voltage of 12V or more, the maximum available output current in this configuration is approximately 0.35A. At lighter loads, the minimum input voltage required drops to approximately 4.7V". So it seems a higher input voltage should work better (like 19V from laptop supply).

And: "the output capacitor values must be larger than what is normally required for buck designs. Low input voltages or high output currents require a large value output capacitor (in the thousands of micro Farads)".

And: "The recommended range of inductor values for the buck-boost design is between 68 μH and 220 μH". So:

(..) and a bigger coil(i dont know the value )

Doesn't cut it, and I'd guess this might be your problem. Amperage is something you could estimate (or feel if coil gets hot), but you have to know what inductance value you're working with. If the value is too high, then the coil takes too long to respond to voltage changes. If the value is too low, in the time the coil is switched on, current rises above maximum, the 'inductive resistance' falls away & current rises faster still. In effect, a short circuit. At which point surrounding parts are very easily fried. You sure the diode & LM2575 are still okay?

Also what confuses me is that the same materials for the +5v dc converter works pretty good.

Well it's a step-down regulator, not an inverting one. So you can use it as such, but it was designed for another job.

 

[BradtheRad]

I don't know specifically how the IC works. Just adding in case it helps...

The coil must be switched at a suitable frequency. It has to happen just at the points when the coil is charged and discharged.

Operating a coil at too high a frequency will prevent adequate current flow through it.

Operating it at too low a frequency will lead to overmuch current flow, possibly frying it.

More henries goes with smaller current flow.

Few henries goes with larger current flow.

Large cap goes with larger current. Small cap goes with smaller current. (Notice this is the opposite from how it is with inductance.)

I don't know if your IC detects the proper switch times automatically, or whether you have to tailor adjust other component value to suit your coil.

 

[infernal669]

Thanx all for your replies. I made some tests with some materials i already have..
Heres a pic with the coils i have. The right one is supposed to be 100μΗ. The other two, i really dont know. The output capacitor is 2200μF. The only way my circuit works is when i use the left one and for input 12V DC~800mA and it gives me -15V output. For 20V~4A input it doesn't work. Now if I use any other of my coils with 20V~4A input i also get nothing..And if i use the 12V DC~800mA input coil is starting to get fried up.